Electrooptical system



May 26,1936. A D, DQWD 2,041,822

l y ELECTROOPTICAL SYSTEM l Filed March 6, 1934 l F/G. 2

FREQUENCY 4 MP. :1 Rear Aaoowo Y Patented May Z6, 1936 PATENT OFFICEELECTROOPTJICAL SYSTEM Andrew D. Dowd, Montclair, N. J., assignor toBell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application March 6, 1934,Serial No. 714,321

' 3 Claims. (Cl. 178-5) This invention relates to electro-opticaltransmission and more particularly to the production of a signalmodulated carrier current suitable for transmission over telephonecircuits.

Where a light beam after being modulated or caused to pulsate at acarrier frequency rate is used to scan a subject and hence its intensityis also modulated in accordance with the varying tone values ofsuccessive elemental areas of the subject and is then 'applied to alight sensitive electric means; the latter is activated by I recting thephase distortion of the line.

the received light to cause the' production of a signal modulatedcarrier current consisting of an unmodulated current component ofcarrier frequency and two side bands, and in addition a unidirectionalcurrent varying as the tone values of successive elemental areas of thesubject.

Transmission of the unidirectional or'picture Y current is undesirable,for the reason that at the receiving station it causes distortion of theimage or picture. This current consists of components of differentfrequencies represented by a Fourier series which extends from zerofrequency to an upper ylimiting frequency, determined by vthe number ofelemental areas into which the picture is divided and the rate ofscanning. The picture current includes a band of current componentsextending from zero to so-me value, less than the said upper limitingfrequency, which are essential to the production of a picture of goodquality, as well as a series of components of relatively largeamplitudes which are not essential for this purpose.

For eflicient transmission of a picture current band over telephonelines it is desirable to restrict both the upper and lower frequenciesof the band to be transmitted, in order to avoid high line attenuationand also difficulties in cor- As will be set forth below, the carrierfrequency, which is placed near the upper limit of the transmitted band,should preferably be as high as possible to avoid the distortionintroduced by the overlapping currents of diiferent frequencies whichresult from scanning the picture and similar components included in thelower side band which resuits from modulating the carrier current withthe picture currents. For this reason it is desirable to use a carrierhaving a frequency substantially equal to twice the limiting frequencyof the essential picture band and to select the lower side band of themodulation products without seriously distortingany of the selectedcomponents, otherwise the received picture will be distorted. However,the lower side band and the picture current include a series ofcomponents having the same frequencies, i. e., certain currentcomponents of the picture band overlap their counterparts in the lowerside band. This condition presents a serious operating difliculty, sinceit is impossible to provide selective means which, while freely passingthe desired compo- Vnents of the lower side band, can at the same timesuppress these overlapping components, the presence of which causedistortion of the pic ture.

The present invention relates to a system of the type described above,having a transmitting apparatus adapted to produce a picture or imagelmodulated carrier current which may be efficiently transmitted overtelephone circuits and includes means for substantially eliminating orso controlling the overlapping components that the receiver may operateto produce a high quality picture or image of the subject scanned at thetransmitter.

An object of the invention is to control the rate of scanning of asubject and the frequency of modulation of the scanning beam in such amanner as to cause the production of a single side bancl'picturemodulated carrier, that may be efficiently transmitted over telephonecircuits and used at the receiver to control the production of asubstantially undistorted image or picture. l

Another object is to control the production of a picture or imagemodulated carrier current which may be eiiiciently transmitted overtele-u phone circuits and may be used to produce a substantiallyundistorted reproduction at the receiver by establishing and maintaininga xed relation between the picture or field scanning rate and theVfrequency of modulation of. the scanning beam at the transmitter.

An auxiliary object is to effect the above mentioned objects bycontrolling the speed of the scanning member and the rate of Vibrationof the light modulating element, respectively, by alternating currentsof different frequencies, the frequencies of which are controlled by analternating current of constant frequency.

One embodiment, selected for illustration, comprises a picturetransmitting system in which the picture to be transmitted is mounted ona rotating cylinder having associated therewith an optical system,including a light source, means for concentrating the light supplied bythis source into a, beam, a two-ribbon light valve for modulating thebeam of light at a carrier frequency rate and a light sensitive electricdevice, such system being mounted on a carriage which is continuouslymoved at a uniform rate longitudinally of the cylinder; whereby, due torelative motions of the cylinder and optical system, the successiveelemental areas of the picture are illuminated by the moving light beamand light reflected from the picture is directed to the light sensitivemeans which is thereby activated to cause the production of a picturemodulated carrier current including an unmodulated component of carrierfrequency and a unidirectional picture current. The light valvecomprises a mounting provided with two fixed jaws extending transverselyof the direction of travel of the picture and its two ribbons aremounted under `tension in `a magnetic field of constant intensity andsupplied with alternating current of a carrier frequency, whereby theyare caused to move bodily toward and away from each other in a directiontransversely-of the fixed jaws and hence transversely of the `motion ofthe picture, -i. e., the direction of scanning. This method of vibratingthe ribbons results in the production of a unidirectional or picturecurrent in which the frequency band width occupied by the currentcomponents essential to the production of a satisfactory picture at thereceiver is materially narrowed and the amplitudes of the non-essentialcomponents are smaller `than in apicture current produced when'theribbons vibrate in a direction lengthwise of the direction of scanning.

Filtering means are used to Yselect the components of the lower sideband of the modulation products which are essential vto pictureproduction, to suppress the upper side band and the unidirectionalpicture current. Because of the curved characteristic of the cut-01T vofthe filter, certain components of the upper side band as well as certain-components of the picture current :and the selected Yside band, whichare not essential to the production of Aa good quality picture at thereceiver, will pass through the filter.

The motor driving .the cylinder carrying the picture at the transmitteris controlled by an alternating current from a source which'also`supplies current to a harmonic generator, one harmonic componentsupplied by this generator is selected and used to control the frequencyof current produced by a source, which supplies current of carrierfrequency to the. vibrating ribbons of the light valve included in thepath of the scanning beam; whereby the rate of scanning the picture andthe frequency of modulation or pulsation vof the light beam Vare iixedlyrelated, with the result that the distortion, caused by thenon-essential components of the picture current and selected yside bandthat are transmitted by the filter, is materially reduced and hence thequality of the picture produced at the receiver is greatly improved.

A detail -description of theinvention follows and is illustrated in theattached drawing in which,

Fig. l illustrates more or less Vdiagramrnatically a system involvingthe invention;

Fig. 2 illustrates the selective characteristic of the filter;

Figs. `3 and 4 are sketches of a pilot or guide strip which will behereinafter referred to.

Referring to Fig. l there is shown a picture transmitter T connected toa receiver VR. by a telephone line L. The transmitter comprises Yacylinder I driven through a clutch 2 by a constant speed direct currentmotor 4, having associated therewith a sending control means suppliedwith alternating current from a constant frequency source 5. The motoris designed to provide a uniform driving torque and within its housingand on the same shaft as its armature is an inductor type generator 6which furnishes an alternating current of fixed frequency when driven atnormal speed by the motor. For illustratlve purposes the normal speed ofthe motor may be revolutions per minute and the frequency of thealternating current supplied by the generator Ii when the speed of themotor is normal may be 300 cycles, which is the same. as

that supplied by the constant frequency source 5. The difference inphase between the current supplied by the vsource 5 and that supplied bythe generator 6 is used to maintain the speed of the motor 4 constant.vThe sending control unit, comprising a vacuum tube phase detector 'Iand an amplifier 8 which is adapted to be controlled by the phasedifference between these. frequencies and operates to vary the armaturecurrent supplied to the motor. The phase detector 'I consists'of twovacuum tubes, the grids of which are supplied with current from theconstant frequency source I5, while a voltage from the inductor:generator 6 .is applied to the plates of these tubes. The outputcurrent from the detector 'I, which will fdepend upon the phase relationAbetween the two applied voltages, is supplied to the control grids ofthe amplifier 8, consisting of four vacuum tubes connected in parallel,which is 'capable of supplying the total armature current of the motor.The fields of the motor and inductor generator are supplied from abattery (not shown) and the motor armature is supplied through theVariable impedance of the plate circuits of amplifier 8. For a completedisclosure of the motor control unit briefly described above, referencemay be made to application Serial No. 670,261 of E. R. Morton, filed May10, 1933.

Connected tothe motor through a gearing 9 is a lead .screw II) fordriving a carriage I I provided with a housing enclosing an opticalsystem comprising a source of light of constant intensity I3, a lightvalve I4, a small mirror m and a light sensitive electric means I5. Thelead screw I 0 operates to move the carriage longitudinally of therotating cylinder I at a uniform constant speed. The light valve I4comprises two flat ribbon conductors stretched under tension and forminga loop circuit which is located in a magnetic field obtained from apermanent magnet. The direction of the magnetic field is at right anglesto the plane containing -the two largest dimensions of the ribbon, i.e., length and breadth, so that, when an alternating current passesthrough the ribbons, they will vibrate sidewise alternately increasingand decreasing the separation between them. These ribbons lie ina commonplane and each is parallel to the Adirection of motion of the cylinder,and the inner edges of the respective ribbons cooperate with two fixedjaws which are carried by the light valve structure and which extendtransversely to the ribbons to form an aperture. A beam of light fromthe source I3 is directed by a condensing lens I6 through ythe apertureof the light valve and an image of this aperture is focussed by a secondlens system I'I to form an intense spot of light in the form of a squareon the surface of the picture carried by the cylinder I. Alternatingcurrent of the desired frequency is supplied to and passes through thelight valve ribbons whereby they are caused to vibrate and therebymodulate or vary the light supplied by the source I3 at a carrierfrequency rate.

In accordance with this invention, current from the constant frequencysource 5 is supplied to a distorting device I8 which operates to producecurrent components the frequencies of which `are harmonically related tothat supplied by the source 5.

The output circuit of the harmonic generator includes a lter F whichselects one of the harmonic components and suppresses the fundamentallas Well as the other harmonics. The selected harmonic is impressedacross a resistance 60 included in the input circuit 6I of a vacuum tubeoscillator, Which comprises this input circuit connected between itsgrid or control element and filament and includes a Winding 62 which iscoupled With a similar Winding 63, shunted by an adjustable condenser 64included in the output circuit of the tube. The coil 63 and condenser 64constitute a tuned circuit, included in one alternating current branchof the filament to plate or output circuit of the tube, Which in generalserves to determine the frequency ofthe oscillations generated andshould be adjusted to the desired frequency, i. e. approximately thefrequency of the harmonic. Space current issupplied from asource througha choke coil 66 tothe plate of the oscillating tube and oscillatingcurrent is supplied through the transformer 6l to the circuit includingthe ribbons of the light valve I4.

The harmonic component impressed across the resistance 66 coacts withthe current generated by the oscillator and the effect of this coactionis to pull the oscillator rinto step with the frequency of the impressedharmonic and thereby cause the frequency of the oscillations and theharmonic current to become identical. In this case, the output currentfrom the oscillator is supplied to and passes through the light valveribbons which are thereby caused to vibrate at the frequency of theharmonic and in synchronism with the scanning operation.

Due to the relative motion of the drum or cylinder and the opticalsystem, the spot of light, fluctuating at a'carrier frequency rate scanssuccessive elemental areasof the picture mounted on the drum and thedoubly modulated light reflected from these vareas is applied to thelight sensitive electric device I5, which is thereby activated to causethe production of a unidirectional picture current and a picturemodulated carrier current consisting of an unmodulated carriercomponent'and two side bands. The, unidirectional picture currentcomprises a continuous series of components having relatively largeamplitudes, which constitute a frequency band, eX- tending from Zerofrequency to an Vupper limiting frequency, which is largely determinedby the number of elemental areas into which the picture is divided andthe rate of scanning,` and eachV side band of the modulated carriercurrent comprises a similar band extending in both directions in thefrequency spectrum from the carrier frequency, which is determined bythe rate of vibration of the light valve ribbons. The small mirror mdiverts carrier modulated light directly to the light sensitive meansI5, which is thereby activated to cause the production of current ofcarrier frequency in phase With the unmodulated component of the picturemodulated carrier current, toprovide carrier bias, whereby the energylevel of the carrier current is raised at the transmitting `terminal tominimize the eect of disturbances entering by Way of the line.

By positioning the light valve ribbons so that they lie parallel to thedirection of mo-tion of the picture, i. e., parallel to the direction ofscanning, and causing them to vibrate transversely thereto, thedimension of the aperture and hence of the spot of light illuminatingthe elemental areas of the picture along the length of the scanning lineremains fixed, While its dimension transversely of the scanningdirection varies in accordance with the Wave shape of the alternatingcurrent ovving through the ribbons. This results in a material reductionin the frequency band Width occupied by the current components ofrelatively large amplitude present in the unidirectional current .and inthe side bands and also in a reduction in the amplitudes of certain ofthe selected components. For a complete disclosure of this feature,reference may be made to application Serial No. 713,939, of F. W.Reynolds, filed March 3, 1934.

The light sensitive means I5 is supplied with current from a directsource 2 I, and the photoelectric current produced thereby is suppliedby a transformer 22 to an amplifier 23. The output current from thisamplifier is supplied to a band lter 24 a-dapted to select the lowerside band of the picture modulated carrier current which is thenamplified to the desired energy amplified by the amplier 28 an-d isrectified or demodulated by a full Wave rectier 29 to yield the picturecurrent, which is selected by the filter 36 and supplied to the ribbonof the light valve 3l included in an optical system mounted on acarriage 32. Theroptical system comprises a light source of constantintensity 33, a lens 34 for concentrating light supplied by the source33 on the light valve aperture and a lens system 35 adapted to image theaperture of the light valve on a photographic film mounted on a rotatingcylinder 36.

Cylinder 36 of a lead screw 3l, adapted to move the carriage 32longitudinally of the cylinder at a uniform rate, are connected by meansof' an electromagnetically operated clutch 38 to a driving motor 39which is controlled in the same manner as the motor 4 at thetransmitter, i. e., by means of a phase detector 'I' in cooperation withan inductor generator 6 on the motor shaft and a constant frequencysource 5', and an amplifier 8 which serves to supply armature current,controlled by the phase relation of the currents supplied to thedetector l', to the motor 39.

Y Due to the displacement of the ribbon of the light valve 3l undercontrol of the received picture current and the relative motions of thecylinder and carriage, varying amounts of light, corresponding to thevarying tone value of successive elemental areas of the picture scannedat the transmitter, will be supplied to the film, and hence a picture ofthe subject scanned at the transmitter will be recorded, provided thetransmitting and receiving cylinders are operated in synchronism and inphase with each other. Synchronous operation of the cylinders is assuredby providing sources 5 and 5 which generate alternating currents of thesame frequency which do not vary by morethan about one part in fivehundred thousand and byl causing the cylinders to be simultaneouslystarted by a control switch at one of the cooperating stations.

Starting is effected by actuating the start key 40 at the transmittingstation, which operates to simultaneously control the supply of currentto the electromagnet controlling the clutch at the transmitter and to asimilar electromagnet at the receiver through the clutch control 4I.Starting may be controlled equally well by the receiving operator, inwhich event, the start key could be placed at the receiving station.

Since transmission of the low frequency picture currents over the lineis impossible, because of the line characteristics, it is necessary tocombine these low frequency variations with a carrier in order that theymay be transmitted over the line. The lter 24 is used to select, fromone of the side bands, resulting from this modulation, a series ofcomponents essential to the production of a satisfactory picture at thereceiver. The selected components constitute a band extending from theunmodulated carrier component to a lower limiting frequency. Since it isnecessary to pass freely all of the side band components essential tosatisfactory picture reproduction, the cut-off of filter 24 must be suchthat it does not attenuate any of the desired components, otherwisedistortion will result.

In order to obtain satisfactory definition in the received picture whileremaining within limits prescribed by the transmission characteristicsof the telephone circuits, the carrier frequency was made slightlygreater than twice the frequency of the picture modulated current bandessential for satisfactory reproduction, or, in other words, twice thefrequency of the band passed without attenuation by the filter, and theribbons of the light valve were supplied with oscillations of thisfrequency.

Fig. 2 illustrates the selective characteristic curve of lter 24 andlines U and S respectively indicate the frequency range of the picturecurrent band and lower side band which include components of appreciableamplitudes. As therein indicated these bands overlap and the filtertransmits a frequency band extending from the carrier frequency to apoint A in the frequency spectrum and a further band extending from A toB which includes the overlapping components of the picture band and ofthe side band, as well as a limited number of the upper side bandcomponents. The filter is designed in accordance with the disclosure inNyquist Patent 1,748,168, issued February 25, 1930 and hence operates tocause the selected components of the upper side band to be in phase withcomponents of corresponding frequencies in the lower side band whichwould otherwise be subject to distortion.

Although the overlapping components of the unidirectional current andthe lower side band passed by the filter are materially attenuated, theymay cause appreciable distortion of the picture produced at thereceiver.

Distortion due to the two groups of overlapping components wasparticularly noticeable when the ribbons of the light valve I4 weresupplied with oscillations of the desired constant frequency from anindependent source, i. e., from a source which was not synchronized withthe picture scanning rate. The character of the distortion therebyproduced is shown in Fig. 3, which is a draftmans reproduction of anenlargement of a portion of a pilot or guide strip consisting of aseries of straight and slightly converging black lines on a whitebackground. This strip was applied to the picture surface at thetransmitter and formed a part of the field which was scanned fortransmission. As shown in this figure, the reproduced lines are neitherstraight nor uniformly black, but rather wavy and the successiveelemental areas are not of the same tone value, thereby indicating thatcertain of the elemental areas are not properly aligned With respect toothers and the tone values of some of them are modified. In effect thepattern produced looks like a checkerboard or basket weave in whichdifferent colored strands are used. Distortion of the type illustratedis caused by the interaction of the two groups of overlappingcomponents, one group belonging to the picture current resulting fromthe scanning operation and the other to the lower side band resultingfrom the modulation process in the photoelectric cell. Such interactionresults in distortion of the tone value, and the change in tone valuewill occur in varying positions in each successive revolution of thepicture wrapped around the cylinder, because of the changing relation ofthe two overlapping groups of frequencies resulting from the unsynchronized state of the carrier frequency and the frequency of rotationof the cylinder. This effect is further aggravated by some unavoidablephase distortion in the transformers and filters of the system causingcertain frequencies to be transmitted faster or slower than is demandedby distortionless transmission.

A still further cause of the irregular pattern is the fact that the timeof opening and closing of the light valve is not negligible incomparison with the speed of rotation of the cylinder. This results inthe displacement of tones during successive revolutions of the picturecylinder when the opening and closing of the transmitting light valve isnot synchronized with the rotation of the cylinder. Synchronization ofthe carrier fre quency and the rotation of the cylinder does noteliminate distortion of tone values referred to above, but it controlsthe location of distorted picture elements and hence makes them lessobvious in the received picture. The improvement in the received pictureresulting from the synchronizing operation is illustrated in Fig. 4.This is a draftsmans reproduction of an enlargement of the receivedpilot strip and comprises a series of straight black lines convergingslightly, as in the original scanned at the transmitter. While the edgesof the black lines are not sharply dened but somewhat fuzzy, it isclearly apparent that synchronizing the picture scanning means and thelight modulating means has improved the appearance of the receivedpicture by preventing the irregular distribution of the pictureelements.

What is claimed is:

1. An electro-optical system comprising means for producing a beam oflight, an alternating current source for varying the amount of light ofsaid beam in accordance with a denite law, a eld of view to be scannedby said light beam, means for causing relative motion of said scanningbeam and said eld, means controlled by light received from said eld ofView to produce a picture current and a picture modulated carriercurrent which respectively include current components of the samefrequencies, means for selecting a picture modulated sideband which alsoselects at least some of said components of said picture current, andmeans for producing a definite relation between the frequency of saidalternating current and the relative motion if said i'leld and saidscanning beam.

2. In an electro-optical scanning system comprising an image eld, asource of steady light supplying a beam of light, a light valvecontrolled by an alternating current for cyclically modulating saidlight beam, means for producing relative movement between said lightbeam and said field to cause the former to scan successive elementalareas of the latter, means controlled by light received from said eld ofvieW to produce a picture current and a picture modulated carriercurrent which respectively include current components of thesamerifrequencies, means for selecting a picture modulated sidebandwhich also selects at least some of said components of said picturecurrent, means for generating tWo electric currents having frequenciesbearing denite frequency relationship to each other, and means forapplying these currents to control respectively the alternating currentsupplied to said light valve and to the means for producing relativemovement between the image eld and the scanning beam.

3. An electro-optical system comprising means for producing a carriermodulated light beam,

means for effecting relative movement of said beam and a field of View,means controlled by light received from said subject to produce apicture current and a picture modulated carrier current whichrespectively includes current com'- ANDREW D. DOW'D.

